Add reportedClientSessionId to Rust session create/resume wire

[em-gong]

What

Threads a new optional reported_client_session_id through the Rust SDK's SessionConfig and ResumeSessionConfig, serialized as a flat top-level reportedClientSessionId on both the session.create and session.resume JSON-RPC wire.

Part of a 4-repo fix for CCA (Cloud Copilot Coding Agent) per-session AI-credit attribution. On steering/resume runs the wrong session id lands on X-Client-Session-Id, so AI-credit spend bills to the task session instead of the steering session. This field lets callers seed the runtime's client-session scalar (→ X-Client-Session-Id + telemetry) without touching the logical/registry session_id, which keys resume-reattach, permission hooks, and the workspace filesystem.

Semantics

• Wire key (camelCase JSON): reportedClientSessionId — flat top-level, sibling of sessionId (not nested), on both create and resume.
• Rust field: reported_client_session_id: Option<String>; builder: with_reported_client_session_id(impl Into<String>).
• #[serde(skip_serializing_if = "Option::is_none")] ⇒ absent/None emits nothing and the wire is byte-for-byte identical to today; the engine then falls back to session_id. Non-empty wins.

Flat top-level is required because the runtime consumer (copilot-agent-runtime#16469) forwards this via its standalone-jsonrpc UPDATE_OPTIONS_FORWARDED_KEYS allowlist, which reads a flat top-level param on both create_session and resume_session. Nesting it under a sub-object would break pickup.

Changes (Rust only)

• SessionConfig + ResumeSessionConfig: new reported_client_session_id field (docs; Default/new initialize to None; included in the custom Debug impls).
• with_reported_client_session_id(..) builder on each (mirrors with_client_name).
• SessionCreateWire + SessionResumeWire: new field, skip_serializing_if = Option::is_none, camelCase, flat top-level sibling of sessionId.
• Mapped through both into_wire().
• Tests: absent ⇒ reportedClientSessionId omitted; present ⇒ appears top-level on create and resume (with sessionId unchanged).

Scope: Rust-only (TS intentionally out of scope)

CCA drives copilotd via copilotd bootstrap env vars → copilot-host (Rust) → this Rust SDK builder → closed wire. It does not convey this identity through the TS CopilotClient.createSession/resumeSession JSON-RPC param path, so a TS reportedClientSessionId option would be permanent dead surface for this use case. Any non-CCA TS parity is a separate future PR.

Consumer / release ordering

• Runtime consumer: copilot-agent-runtime#16469.
• This SDK must release before copilot-host can bump its pin and call .with_reported_client_session_id(..).

Validation

• cargo test --all-features — new tests pass.
• cargo +nightly-2026-04-14 fmt --check — clean.
• cargo clippy --all-features --all-targets -- -D warnings — clean.